1. Field of the Invention
The present invention relates in general to a method of reducing the resistivity of a semiconductor film. More specifically, it relates to a method of activating a P-type compound semiconductor film to reduce the resistivity thereof by way of rapidly heating and cooling the P-type compound semiconductor film.
2. Description of the Related Art
The III-V nitrides of wide-band gap such as GaN, InGaN, AlGaN, AlGaInN, etc. and the II-VI group compounds such as P-ZnSe, ZnMgSe, etc. exhibit excellent semiconductor light emission, and therefore they are widely applied to form semiconductor films for fabricating optoelectronic devices such as Light Emitting Diodes (LEDs) and Laser Diodes (LDs). However, problems and limits have been encountered using such compounds for forming semiconductor films, therefore impeding the progress of optoelectronic industry.
Take gallium nitride (GaN) as an example. Due to a lack of appropriate substrates having lattices matched with gallium nitrides, so far gallium nitride films are formed on sapphire substrates. For this reason, a gallium nitride film has to be formed using the technique of forming a buffer layer on the sapphire substrate in conjunction with the process of Metal-Organic Chemical Vapor Deposition (hereinafter referred to as MOCVD). When the gallium nitride film is doped with P-type impurity such as Be, Mg, Ca, Zn, Cd, etc. during the film""s growth, the P-type impurity easily reacts with the hydrogen in the reaction gas (metal-organic compound gas). Therefore, the gallium nitride film formed on the sapphire substrate inevitably has high resistivity. The P-type gallium nitride film fabricated according to the conventional art generally has a resistivity higher than 105 xcexa9cm and a hole concentration lower than 1012 cmxe2x88x923. Consequently, the application of the gallium nitride film is rather restricted.
I. Akasaki and H. Amano (NAGOYA university, Japan) disclosed a method which activates the P-type gallium nitride film doped with Mg using Low Energy Electron Beam Irradiation (LEEBI) so as to reduce the resistivity thereof. However, the method cannot be effectively applied because the use of LEEBI cannot obtain good throughput due to low rate of activation. Further, this method can only reduce the resistivity of the surface portion of the gallium nitride film.
S. Nakamura (Nichia Chemical Industries, Ltd. Japan) disclosed a method using MOCVD method in conjunction with low-temperature gallium nitride buffer layer to form a P-type gallium nitride film on a substrate. Then, the P-type gallium nitride film is annealed at high temperature according to the conventional art, thereby reducing the resistivity of the P-type gallium nitride film. The annealing process is carried out at a temperature of between 400xcx9c1200xc2x0 C. in the ambience of nitrogen gas. Also, a specific processing temperature between 400xcx9c1200xc2x0 C. is kept for longer than one minute. In fact, to complete the whole annealing process totally takes more than 10 minutes, when considering the time for increasing and decreasing the temperature. However, when P-type gallium nitride films are applied to fabricate heterojunction diodes or light emission diodes, the variations may be introduced to composition of the layers of the P-type gallium nitride films or the impurity of the heterojunction may diffuse due to long duration of the annealing process, thus degrading the optoelectronic performance of the diodes.
Therefore, an object of the present invention is to provide a novel method to overcome the above problems, using rapid variation of temperature to generate driving energy to activate P-type compound semiconductor films for reducing the resistivity thereof.
The present invention achieves the above-indicated object by providing a method of activating P-type compound semiconductor film for reducing the resistivity thereof. The method comprises the following steps.
(a) Grow a first P-type compound semiconductor film; wherein the first P-type compound semiconductor film is made from a III-V nitride or a II-VI group compound doped with P-type impurity.
The first P-type compound semiconductor film can be grown using metal-organic chemical vapor deposition (MOCVD) method, molecular beam epitaxy (MBE) method, or hydride vapor phase epitaxy (HVPE) method.
The III-V nitride can be selected from GaN, InGaN, AlGaN, or AlGaInN, and the II-VI group compound can be selected from P-ZnSe, ZnMgSe, etc.
(b) Heat the first P-type compound semiconductor film at a specific temperature-increasing rate (greater than 50xc2x0 C./sec) thereby rapidly increasing the ambient temperature from an initial temperature to a first specific temperature (not less than 850xc2x0 C.).
(c) Cool the first P-type compound semiconductor film at a specific temperature-decreasing rate (greater than 20xc2x0 C./sec) thereby rapidly decreasing the ambient temperature from the first specific temperature to a second specific temperature (for example, 400xc2x0 C.) such that the first P-type compound semiconductor film is transformed into a second P-type compound semiconductor.
(d) Continuously decrease the ambient temperature from the second specific temperature to the initial temperature.
It is noted that the time duration when the ambient temperature is greater than the second specific temperature is less than one minute during the course of temperature variation (steps (b) and (c)), and the resistivity of the second P-type compound semiconductor is lower than that of the first P-type compound semiconductor.
In addition, an optional step of keeping the ambient temperature at the first specific temperature for a holding time (less than 25 seconds) can be performed between step (b) and step (c).